Microlithography is used for producing microstructured components, for example integrated circuits. The microlithographic process is carried out with a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 4 nm to 6 nm, are currently under development. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements. For the same reason, beam shaping and beam projection should be carried out in a vacuum.
The mirrors may for example be fastened to a supporting frame (force frame) and be configured to be at least partially manipulable or tiltable in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the pm range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be corrected.
For the purposes of displacing the mirrors, in particular in the six degrees of freedom, actuators which are actuated by way of a control loop are assigned to the former. An apparatus for monitoring the tilt angle of a respective mirror is provided as part of the control loop.
For example, WO 2009/100856 A1 discloses a facet mirror for a projection exposure apparatus of a lithography apparatus, which has a multiplicity of individually displaceable individual mirrors. To ensure the optical quality of a projection exposure apparatus, very precise positioning of the displaceable individual mirrors is desired.
Furthermore, document DE 10 2013 209 442 A1 describes that the field facet mirror can be in the form of a microelectromechanical system (MEMS). However, pulsed plasma can be produced in the residual gas in the illumination system due to the photons of the EUV radiation source of the lithography apparatus, and electrons can be liberated from the mirror surfaces of the MEMS mirrors due to the photoelectric effect. This can bring about temporally and spatially varying current flows over the MEMS mirrors of the field facet mirror. These temporally and spatially varying current flows over the MEMS mirrors can significantly disturb the evaluation electronics of the apparatus for monitoring the tilt angle of the respective mirror.